return MoreObjects.toStringHelper(this).addValue(identifier).toString();
  }

  /** Simple logging handler for subscriber exceptions. */
  static final class LoggingHandler implements SubscriberExceptionHandler {
    static final LoggingHandler INSTANCE = new LoggingHandler();

    @Override
    public void handleException(Throwable exception, SubscriberExceptionContext context) {
      Logger logger = logger(context);